Phosphodiesterase 2 (PDE2) inhibitors are promising therapeutic targets for treatment of cognitive impairment in diseases such as Schizophrenia, Alzheimer's disease and depression. Inhibitors of PDE2 have emerged as potential candidates to improve synaptic plasticity and memory function.
Phosphodiesterases (PDE) are expressed in nearly all mammalian cells. To date eleven families of phosphodiesterases have been identified in mammals. It is well established that PDEs are critically involved in cell signalling. Specifically, PDEs are known to inactivate the cyclic nucleotides cAMP and/or cGMP.
PDE2 hydrolyses both, cGMP and cAMP. It is both abundantly expressed in the brain indicating their relevance in CNS function.
The expression of PDE2 in the hippocampus, the cortex and in the striatum indicate an involvement in the mechanism of learning and memory/cognition. This is further supported by the fact that increased levels of both cGMP and cAMP are involved in the process of short and long term potentiation (LTP) forming. Further data support the procognitive effect of PDE2 and a synergistic effect of PDE2 on cognition. Furthermore, the expression of PDE2 in the nucleus accumbens (part of the striatum), the olfactory bulb, the olfactory tubercle and the amygdala supports additional involvement of PDE2 in the pathophysiology of anxiety and depression. This is supported by in vivo studies.
It is commonly accepted (free drug hypothesis) that unbound or free drug concentration at the site of action is responsible for pharmacological activity in vivo at steady state and, in the absence of active transport, the free drug concentration is the same in any biomembrane.
For drugs with an intended action in the central nervous system (CNS), it is assumed that unbound drug in interstitial spaces (ISF) in the brain is in direct contact or in equilibrium with the site of action. Because cerebrospinal fluid (CSF) is in direct contact with the brain tissue, it is assumed to readily equilibrate with brain interstitial fluid concentration so that CSF concentration is used as a common surrogate measure for drug unbound concentration in pre-clinical pharmacology studies. Accordingly, for compounds with an intended action in the central nervous system it is important that they reach a high CSF concentration and a high CSF to plasma ratio in order to have high pharmacological activity in the CNS.
At steady state and in the absence of active transport, the unbound brain concentration can also be estimated with the experimentally more accessible unbound plasma concentration by measuring the plasma protein binding (PPB) across species.
High membrane permeability and absence of active transport process at the BBB (blood brain barrier) togheter with plasma/brain tissue binding are recognised as the primary determinant of drug disposition within CNS.
High metabolic stability is desirable in order to achieve significant exposure of a drug within the body.
Several families of PDE2 inhibitors are known. Imidazotriazinones are claimed in WO 2002/068423 for the treatment of e.g. memory deficiency, cognitive disorders, dementia and Alzheimer's disease. Oxindoles are described in WO 2005/041957 for the treatment of dementia. Further inhibitors of PDE2 are known from WO 2007/121319 for the treatment of anxiety and depression, from WO 2013/034761, WO 2012/104293 and WO2013/000924 for the treatment of neurological and psychiatric disorders, from WO 2006/072615, WO 2006/072612, WO 2006/024640 and WO 2005/113517 for the treatment of arthritis, cancer, edema and septic shock, from WO 2005/063723 for the treatment of renal and liver failure, liver dysfunction, restless leg syndrome, rheumatic disorders, arthritis, rhinitis, asthma and obesity, from WO 2005/041957 for the treatment of cancer and thrombotic disorders, from WO 2006/102728 for the treatment of angina pectoris and hypertension from WO 2008/043461 for the treatment of cardiovascular disorders, erectile dysfunction, inflammation and renal failure and from WO 2005/061497 for the treatment of e.g. dementia, memory disorders, cancer and osteoporosis.
Benzodiazepine like PDE2 inhibitors are described in WO 2005/063723 for the general treatment of CNS diseases including anxiety, depression, ADHD, neurodegeneration, Alzheimer's disease and psychosis.
Newer PDE2 inhibitor families are described in WO 2015/096651, WO 2015/060368 and WO 2015/012328.